Electrodeposition of molybdenum



United States Patent O 3,369,978 ELECTRODEPOSITION OF MOLYBDENUM Charlie Wyche, Washington, D.C., and Frank X. Mc-

Cawley, Cheverly, and David Schlain, Greenbelt, Md., assignors to the United States of America as represented by the Secretary of the Interior No Drawing. Filed Oct. 1, 1964, Ser. No. 400,963 10 Claims. (Cl. 204-64) ABSTRACT OF THE DISCLOSURE Electrodepositing elemental molybdenum from a fused bath composed of a mixture of (1) borates of alkali metals or alkaline earth metals, (2) molybdates of alkali metals or alkaline earth metals, and (3) molybdenum oxide.

The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

Molybdenum is becoming increasingly important as a structural material in industry, in high speed aircraft, and in missiles because of its relatively high strength at temperatures from 1800 to 2500 F. and its relatively low cost. It oxidizes readily at temperatures over 1000 F. However, it is resistant to nonoxidizing mineral acids, to nonoxidizing gases and to liquid metals. An electroplating technique would make it possible to replace molybdenum castings or forgings with-parts plated with molybdenum, thus making it possible to bypass difficult fabrication techniques now in use and to take advantage of favorable properties of underlying metals.

Molybdenum has previously been deposited from fused salt electrolytes but the deposits obtained were thin and their quality poor. In addition, the fused salt baths were often extremely corrosive to both electrodes and furnace parts.

It is, therefore, an object of the present invention to provide a fused salt bath from which pure, dense, wellconsolidated and adherent coatings of molybdenum can be deposited on a variety of basis materials.

It is a further object to provide such a bath that is stable and usable at high temperatures for extended periods of time, as well as being relatively noncorrosive.

It has now been found that the above objects may be achieved by means of a fused salt electrolyte comprising an alkali metal or alkaline earth metal borate and an alkali metal or alkaline earth metal molybdate or molybdenum oxide.

Lithium and sodium metaborates have been found to be particularly effective in the invention; however, other borates of the alkali or alkaline earth metals such as tetraborates, orthorbora tes, pentaborates and diborates may also be used. Also, borates of other alkali metals (K, Rb or Cs) or alkaline earth metals (Be, Ca, Sr or Ba) may be used.

Lithium and sodium molybdates and M have also been found to give good results. However, again, molybdates of other alkali metals or alkaline earth metals (as listed above) maybe used. Also, other oxides such as M00 M 0 or M0 0 could be used in place of M00 3,369,978 Patented Feb. 20, 1968 "ice When metaborates are used, the weight ratio of borate ion (B03) to molybdenum should be from about 10:1 to 300:1. Quality of the coating usually begins to decrease when the borate to molybdenum ratio is about 20:1 and at a ratio of 15:1 the coatings will usually include both adherent molybdenum metal and fl-at nonmetal lic crystals. When the ratio is 9:1 adherent metal is usually not deposited and large amounts of nonmetallic crystals appear in the electrolyte.

At the other extreme, when the ratio is above about 75 :1 the deposit is partly adherent, consolidated metal and partly nonadherent metallic powder. At a ratio of about 98:1 the consolidated deposit usually has only about 79 percent cathode current efiiciency. At a ratio of 300:1 or higher the entire deposit is usually in the form of a nonadherent powder.

When borates other than met-aborates are used optimum proportions may vary somewhat from the above and are best determined experimentally. Optimum proportions may also vary somewhat with different molybdenum compounds.

Care should be taken to exclude moisture from the electrolyte. In the examples below the metaborate salts were dried in a vacuum oven at 200 C. and at a pressure of less than 5 microns of mercury, and then fused. The molybdates were also fused and the M00 was dried at 600 C. in an inert atmosphere. Mixing of the salts was carried out in a dry chamber.

The electrolysis is carried out in a conventional cell comprising cathode, anode and the fused salt electrolyte. In the examples below, the cell was enclosed in a sealed, inert atmosphere furnace. The anode and cathode were pure molybdenum in the form of rectangular sheets and the direct current was maintained at about 110 amperes per square foot. The cathode was rotated during electrolysis to agitate the electrolyte and to depolarize the electrodes. At the end of the electrodeposition period the deposits were removed from the bath and allowed to cool to room temperature in the inert atmosphere.

Optimum temperature of the bath will vary according to the specific composition of the bath and the type 1 of deposition process (electroplating, electrowinning, etc.).

For good deposits the best range of temperatures is usually about 850 to 925 C.

Cathode current density for satisfactory molybdenum coatings varies from about 1 to 150 amperes per square foot. Optimum current densities are about 7 to amperes per square foot. With the use of periodic reverse plating, however, the current density may be extended about tenfold. For deposition processes other than electroplating current densities may range as high as 3,000 amperes per square foot. Atmospheric pressure is employed.

Although deposition in the examples was on a molybdenum cathode, deposition of molybdenum on other substrates such as iron, nickel, copper, Inconel, beryllium, chromium, cobalt, graphite, stainless steel, etc., may be accomplished by the method of the invention by employing the desired metal substrate as cathode.

The method of the invention may be used for electroplating (coating), electrowinning, electrorefining or electroforming. For electroplating and electroforming, the anode should be of pure molybdenum. In the case of elec- 3 troplating the object to be coated is made the cathode while for electroforming the cathode is a metal removable by dissolution (such as copper) or by some other stripping method.

For electrorefining the anode would be the impure molybdenum and the cathode could be molybdenum sheet or other metal. In electrowinning the anode would be made of insoluble material such as graphite or carbon, the cathode of molybdenum sheet or other metal and the electrolyte would be replenished with additions of molybdenum oxide or molybdate.

Obviously many other variations may be made without departing from the essence of the invention as set forth in the claims. Any apparatus suitable for electrodeposition can be used, the electrodes may vary widely in size and shape and a wide variety of electrodeposition processes may be carried out efiiciently and with a resulting high quality deposit of molybdenum.

The following examples will serve to more particularly illustrate the invention.

Example 1 The electrolyte contained 277.6 grams of sodium metaborate, 209.85 grams of lithium metaborate, 49.3 grams of sodium molybdate, 4.17 grams of lithium molybdate, and 303.45 grams of molybdenum trioxide. The metaborate to molybdenum ratio was 52:1. The electrolyte was maintained at a temperature of 900 C. in an argon atmosphere during the electrodeposition period. The cathode was rotated at 250 r.p.m. and the current density was approximately 60 amperes per square foot. At the end of 4 hours the electrodes were raised to a position above the bath, cooled in the inert atmosphere, removed from the furnace tube, washed in warm water, and dried. The anode current efliciency was 103 percent and the cathode current efiiciency was 98.5 percent. The molybdenum deposit on the cathode was adherent and coherent, and it consisted of fine and coarse crystals. Coatings made from the same batch of cell solution and under the same conditions after the initial deposit consisted of fine crystals, and the deposits were smoother. All coatings were adherent, coherent, and bright. The cathode current efliciencies were 98 to 99 percent and the thicknesses of the deposits were 4 to 6 mils.

Example 2 An experiment was conducted with an electrolyte which contained 157.7 grams of sodium metaborate, 119.3 grams of lithium metaborate, 9.06 grams of sodium molybdate, 7.65 grams of lithium molybdate, and 6.33 grams of molybdic oxide. This electrolyte had a borate to molybdenum ratio of 16:1. The preparation of the electrolyte, the conditions of electrodeposition, and the treatment of the deposit were the same as described under Example 1. After 2 hours, the deposit consisted of an adherent, smooth, bright plate of molybdenum partially covered by a mass of dark, flat, nonmetallic crystals. The weight of the consolidated molybdenum deposit was equivalent to a cathode current efiiciency of about 85 percent. The anode current efiiciency was 104 percent. The next eight deposits obtained from this bath became progressively worse as the borate to molybdenum ratio became lower. Each deposit had more of the nonmetallic crystals and less of the adherent metallic plate than the previous one. The current efiiciency for the eighth deposit, which had a calculated borate to molybdenum ratio of 7:1, was only 26 percent and this deposit was completely covered with a heavy mass of flat, dark, nonmetallic crystals.

Example 3 An experiment was conducted which contained 276.65 grams of sodium metaborate, 209.10 grams of lithium metaborate, 2.46 grams of sodium molybdate, 2.12 grams of lithium molybdate, and 1.67 grams of molybdic oxide.

The borate to molybdenum ratio of this electrolyte was approximately 98: 1. After a 4-hour electrolysis period at a current density of amperes per square foot, the cathode deposit consisted of adherent, smooth, bright molybdenum covered With loose, metallic, granular crystals. The cathode current efficiency based on the weight of adherent deposit was 79 percent and the anode current efliciency was 101 percent.

Example 4 An electrolysis was conducted with a mixture of electrolyte which contained 283.54 grams of sodium metaborate, 214.33 grams of lithium metaborate, 0.837 gram of sodium molybdate, 0.707 gram of lithium molybdate, and 0.585 gram of molybdic oxide. The borate to molybdenum ratio of this mixture was about 313:1. After a 4-hour electrolysis period at a current density of 60 amperes per square foot, the cathode was covered with spongy, loose molybdenum. Beneath this spongy material Was :a very thin, well-consolidated deposit of molybdenum with a weight equivalent to a cathode current efiiciency of 0.30 percent. The anode current efiiciency was 102 percent. Several other electrolyses from this bath produced similar deposits.

Example 5 An experiment was conducted with electrolyte which contained 90.87 grams of sodium metaborate, 68.70 grams of lithium metaborate, 8.36 grams of sodium molybdate and 7.07 grams of lithium molybdate. The borate to molybdenum ratio of this electrolyte was approximately 15.5 :1. After a one hour electrolysis period at a current density of 60 amperes per square foot, the cathode had a smooth adherent deposit. The cathode current efliciency, based on the weight of adherent deposit, was 98 percent. It should be noted that the borate content of this bath was too low to give consistently satisfactory coatings. The next deposit was also adherent and smooth. However, in this instance, one edge of the cathode was covered with dark, fiat, nonmetallic crystals and the current efiiciency was only 86 percent. The ratio of borate to molybdenum at the start of this test was 14.8:1. Later deposits from this bath became progressively worse.

Example 6 An electrolysis was conducted with a mixture of electrolyte which contained 90.17 grams of sodium metaborate, 68.31 grams of lithium metaborate and 16.53 grams of molybdic oxide. The borate to molybdenum ratio of this bath was 9:1. After a one and one-half hour electrolysis period at a current density of 60 amperes per square foot, the total cathode deposit consisted of dark, flat, nonmetallic crystals.

What is claimed is:

1. A process for electrodeposition of elemental molybdenum comprising: fusing a mixture of v(l) a metaborate selected from the group consisting of alkali metal and alkaline earth metal borates and (2) a molybdenum compound selected from the group consisting of (a) a molybdate from the group consisting of alkali metal and alkaline earth metal molybdates and (b) a molybdenum oxide to form an electrolyte bath, said bath having a weight ratio of borate ion to molybdenum in the range of about 10:1 to about 300: 1, and electrolyzing the fused electrolyte bath to deposit elemental molybdenum at the cathode.

2. The process of claim 1 in which the weight ratio of metaborate to molybdenum is in the range of about 20:1 to about :1.

3. The process of claim 1 in which the metaborate is lithium metaborate.

4. The process of claim 1 in which the metaborate is sodium metaborate.

5. The process of claim 1 in which the metaborate comprises a mixture of lithium and sodium metaborates.

6. The process of claim 1 in which the molybdenum Referen es Cited ii r iic e fi cl rl f iijwhich the molybdenum UNITED STATES PATENTS conipound is Sodium molybdate 3,075,900 1/ 1963 Zadra et .al 204-64 8. The process of claim 1 in which the molybdenum 5 FOREIGN PATENTS Impound is 777,591 6/1957 Great Britain.

9. The process of claim 1 in which the molybdenum compound is :a mixture of lithium molybdate, sodium HOWARD WILLIAMS Primary Examine rnolybdate and molybdic oxide.

10. The process of claim 1 in which the molybdenum 10 JOHN MACK Exammercompound is a mixture of lithium molybdate and sodium D, R, VALENTINE, Assistant Examiner, molybdate. 

